Probability and Punnett Squares

1. Probability and Punnett Squares

2. Genetics and Probability • Probability – the likelyhood that a particular event will occur • Think of a coin toss – the probability of getting head or tail is 1 chance in 2, or ½ or 50% • The principles of probability can be used to predict the outcomes of genetic crosses

3. Punnett Squares • Punnett Square – a diagram used to determine the probable outcome of a genetic cross • Each box represents 25% probability….

4. Punnett Squares (continued) • Consider a cross between a pea plant that is homozygous for yellow seeds (YY) and a pea plant that is homozygous for green seeds (yy) – these are your parentals Given: Y (big Y) is the dominant allele for yellow; y (little y) is the recessive allele for green

5. Punnett Squares (continued) • Crossing YY x yy, use the square… • The gametes of each parent are shown along the top and sides Y Y y y

6. Y Y Y Y y y y Y y y Y y Punnett Squares (continued) • Next, you write in the possible combinations inside of the square

7. Y Y Y Y y y y Y y y Y y Punnett Squares (continued) • The genotype of the offspring is: 100% Yy • The phenotype is 100% yellow 25% 25% 25% 25%

8. Y y Y Y y Y Y Y y y y y Punnett Squares (continued) • Now self-pollinate your F1 generation to produce the F2 generation (F1xF1=F2) • F1 x F1 is the same as saying Yy X Yy

9. Y y Y Y y Y Y Y y y y y Punnett Squares (continued) • The genotype of the F1 self-pollination is 1 Yy : 2 Yy : 1 yy, same as saying 25% Yy, 50% Yy, and 25% yy, 25% 25% 25% 25%

10. Y y Y Y y Y Y Y y y y y Punnett Squares (continued) • The phenotype of the F1 self-pollination is 3 yellow : 1 green, same as saying 75% yellow : 25% green 25% 25% 25% 25%

11. Probabilities Predict Averages • Probabilities predict average outcome of a LARGE NUMBER of events • Probability cannot predict the precise outcome of an individual event (think of a coin toss) • In genetics, the larger the number of offspring, the closer the resulting numbers will get to expected values